1. Field of the Invention
This invention relates, inter alia, to the field of plasticized vinyl chloride polymers, and more particularly, to such polymers plasticized with a plasticizer composition containing monomeric ester and ethylene-vinyl acetate copolymer.
2. Description of the Prior Art
The use of plasticizers as adjuncts for synthetic thermoplastic resins has for many years been a universal and necessary practice, dictated by the need to modify the performance characteristics of host resins to satisfy particular product service requirements.
The term "plasticizer" is widely acknowledged to refer to those substances which, when added to thermoplastic resins, impart and maintain the flexibility of the resins during the useful lifespan of articles fabricated therefrom. In its larger import, the term "plasticizer" can be understood to include any additive of a thermoplastic resin which significantly modifies a major mechanical property of the resin such as, for example, impact resistance.
One of the principal classes of thermoplastic resins, those belonging to the vinyl chloride polymer group, have been compounded with a wide variety of individual plasticizers and plasticizer mixtures in order to provide a composite blend or polyalloy demonstrating desirable physical and mechanical properties not possessed by the unalloyed polymer. The monomeric esters such as dioctyl phthalate (DOP) and dioctyl adipate (DOA) have figured prominently as plasticizers for polyvinyl chloride resins since they are efficient and impart good low temperature performance characteristics to these polymers. However, these and similar monomeric plasticizers, all being relatively volatile substances, tend in time to diffuse from the host resin with several disadvantageous consequences. Loss and/or migration of plasticizer is necessarily accompanied by a proportionate loss of flexibility of the host resin and the attendant embrittlement of the polymer article results in a curtailed product lifespan. In certain end use applications, a high rate of plasticizer diffusion cannot be tolerated. So, for example, polyvinyl chloride tubing for medical applications must not give off any significant quantities of plasticizer.
It has previously been proposed to employ polymeric plasticizers for vinyl chloride resins for the reason that such materials, being less volatile than the monomeric plasticizers, will have less of a tendency to migrate within, and volatize from, the host resin. U.S. Pat. No. 4,003,963 describes a blend of polyvinyl chloride and polyvinyl alcohol, the latter containing up to 3% by weight of residual vinyl acetate groups. The blends are said to possess enhanced resistance to gas permeability and moisture vapor transmission. U.S. Pat. No. 3,968,184 describes impact-resistant blends of polyvinyl chloride and ethylene-vinyl acetate copolymers on which have been grafted units of aromatic vinyl compounds, (meth) acrylonitrile and aliphatic monoolefins. Similarly, U.S. Pat. No. 3,960,986 describes a blend of polyvinyl chloride and a vinyl chloride grafted copolymer of ethylene and vinyl acetate, but in addition thereto, this patent also calls for the use of a small amount of ethylene-vinyl acetate copolymer. The blends can also contain lubricants such as alkyl esters of long-chained dicarboxylic acids and higher fatty acids. U.S. Pat. Nos. 3,888,948; 3,853,970; and 3,607,980 each relate to graft copolymers of ethylene-vinyl acetate copolymer and vinyl chloride demonstrating improved impact strength. U.S. Pat. No. 3,517,083 describes polyblends of polyvinyl chloride and a solid ethylene-vinyl acetate copolymer as an impact modifier. The polyblends can also contain a processing aid or co-plasticizer such as dioctyl phthalate.
Solid ethylene-vinyl acetate copolymers have limited efficiency as plasticizers for polyvinyl chloride compared with the monomeric plasticizers and must therefore be used at higher levels to achieve a level of plasticization comparable to that provided by the monomeric compounds. Not only does the need to use relatively large amounts of solid ethylene-vinyl acetate plasticizer offset in significant measure the economic advantages of the lower cost polyvinyl chloride base resin, the high concentrations of this plasticizer result in an overall diffusion of polymer from the host resin which is not unlike that of the more effective monomeric plasticizers. Moreover, plasticization of polyvinyl chloride with relatively high molecular weight materials such as solid ethylene-vinyl acetate copolymer tends to impair the physical properties of the polymer at low temperatures. Brittleness and optical haze are frequently encountered with polymer plasticized polyvinyl chloride resins at low temperatures.
While U.S. Pat. No. 2,657,188 describes the plasticization of polyvinyl chloride with liquid ethylene-vinyl acetate copolymer, investigation has shown that this plasticizer does not prevent a sharp reduction in low temperature performance of the polyalloy.
Accordingly, there exists a need for plasticized polyvinyl chloride resins having low cost, low extractible plasticizer systems which do not compromise the performance characteristics of the base resins at low temperatures.